In various technologies, monocrystals of the most diverse substances are utilized, such as doped alkali halogenides of Nal, Csl, NaCl, KCI, KI, for example, as scintillator materials for detecting high-energy radiation, as semiconductors for microelectronics such as silicon or gallium arsenide, or aluminum oxide (sapphire) likewise in semiconductor technology as substrate material. Crystals with greater and greater diameters are increasingly required by the semiconductor industry. An article by U.H. Nestor, Mat. Res. Soc. Symp. Proc. Vol. 16 (1983), p. 77 to 85 teaches the production of sodium iodide crystals with a diameter of 75 cm and a weight up to half a ton, for example.
In most of the currently common growing methods, vessels are required in order to receive the melt of the substance. In the Bridgman-Stockbarger method, the vessels are exactly as large as the desired diameter of the crystal; in the Czochralski method, the vessels are slightly larger than the desired diameter of the crystal. These vessels must consist of a material that is mechanically stable and, in addition, inert relative to the melt. For growing the cited alkali halogenide crystals, vessels of silicate glass, graphite or precious metal are usually used. While platinum demonstrates very favorable properties, platinum requires enormous investment costs due to the required crucible size. A suitable and more favorable material with respect to the material costs is iridium, whose treatment is however more expensive and requires a higher initial outlay. Glass is a cheap material, but far more brittle than most metals. Melting vessels of glass are thus limited in size to 60 cm for reasons of stability, from which only crystals up to 40 cm can be pulled according to the Czochralski method.
Stable melting vessels are necessary for the growing of halogenide crystals, since crucible walls of silicate glass are already corroded by the melt given the presence of slight traces of water or hydroxide molecules. As a result, the melt strongly adheres to the crucible following solidification. The enormous tensile stresses arising in cooling due to the thermal contraction of the melt can also lead to the destruction of the crucible. In the known method for producing halogenide crystals, this problem is solved in that the hydroxide is bonded by reaction with various gaseous substances, preferably through replacement with hydrogen halide compounds, and thus removed from the crystal growing vessel (see for example JP-A 63 107 886). A disadvantage of this method is that it requires a high technical outlay, since the vessel must be evacuated and the reaction gasses must be cautiously added in a metered fashion.
Therefore, there is a need for a device for liquefying and crystallizing substances with which monocrystals with particularly large diameters can be created in a cost-effective fashion in a simple method.